An Overview of Research on the Definition and Formation of Cities

Urbanization, as the essential way to modernize and solve the issue of agriculture, rural areas and farmers, it provides strong support to promote the regional coordinated development. To sort out and summarize the definition and formation of cities can provide some ideas for rural areas to find their own urbanization road.


Introduction
Urbanization is the only way to modernization, an effective way to solve the problems of agriculture, rural areas and farmers, and strong support to promote regional coordinated development. The definition and formation of cities are the basis of urbanization related research. Without a clear definition of the concept of city, the development of the city is hard to implement. And without a comprehensive understanding of the formation of the city, the driving force of urbanization development cannot be fully grasped. So, it is particularly important to clearly define the concept of "city" and to explore the causes of the formation of cities

Definition of City
There are many definitions of city, but there is no final conclusion at present, which mainly include the followings. One view is that city is where has defensive walls. This view obviously confuses "city wall" with "city". At the same time, this opinion neglects the different processes of the emergence of cities in the East and the West. Another standpoint holds that city includes city wall and market, that is, city walls with defensive functions and markets for commodity exchange.
This point ignores the political, religious and other reasons in the forming of cities. Due to city and civilization are derived from Latin which means civitas, some scholars equate city with civilization. Song Junling thought that city is the existence form of civilized human beings and the main carrier of human civilization. However, this view is inconsistent with the fact. For example, the civilization of the Maya lowlands in Central America did not have recognized cities.
With further research, the errors of the above three viewpoints are gradually exposed. At present, more popular viewpoints emphasize the centralization of cities. The Encyclopedia Britannica defines cities a relatively permanent and highly organized place with a concentrated population, which is bigger and more

Standards for the Formation of Cities
As for the formation criteria of cities, scholars at home and abroad have put forward different criteria according to different disciplinary backgrounds. From the perspective of archaeology, British archaeologist Child proposed ten criteria for the formation of cities. Firstly, the population of the city is more denseness than that of the settlement population. Secondly, the population structure is diversified. Thirdly, taxes and surplus wealth are concentrated in the power organs. Fourthly, there are large-scale public buildings. Fifthly, social hierarchical differentiation has emerged. Sixth, there are inventions of characters. Seventh, geometry and astronomy are further developed. Eighth, there are full-time artists. Ninth, foreign trade has emerged. Tenth, the way of living is no longer based on consanguinity [2]. Guliangyev, a Soviet scholar, also put forward eight urban standards from the perspective of archaeology. One is the emergence of rulers and their palaces. The second is the emergence of large temples and religious areas. Third, the most important palaces, temples and buildings are sepa-rated from civilian houses. Fourthly, religious areas are obviously different from residential areas. Fifthly the luxurious mausoleums and tombs. Sixth, large-scale works of art have been produced. The seventh is the formation of characters (inscriptions and stone carvings). Eighth is the number of signs, such as large squares, a large number of residential and public housing, more dense residents and so on. Japanese scholar Qianqiu Ono synthesizes the views of archaeology and history, and divides the criteria for the formation of ancient cities into seven aspects. First is the establishment of primitive state organizations and kingship. The second is the dense population. Third is the differentiation of social classes and the specialization of professions. Fourth is the emergence of large monumental buildings. Fifth is the invention of characters and metal objects and the development of science and technology. Sixthly, emerge of participating knowledge-based activities in spare times because of the production of surplus products. Seventh are the emergence of Commerce and the development of trade organizations [3]. Gao, Songfan put forward three urban standards from the geographical point of view. First is the complex of multi-functions (at least two kinds). Then is the place where population, handicraft industry, trade, wealth, construction and public facilities are concentrated. The last is the high population density, mainly engaged in non-agricultural occupations [4]. Xu, Hong proposed three urban standards from the perspective of archaeology. First, as the center of power of a state, it has the function of political, economic and cultural center in a certain region. The monarch, as a symbol of power, arises from it. In archaeology, it is shown as the existence of large-scale rammed earth construction relics which include ceremonial buildings such as palace base sites, altars, etc.).Secondly, because of the differentiation of social strata and the division of industry, the city has the complex characteristics of residents. The development of non-agricultural production activities has made the city that becomes the first non-self-sufficient society in human history. The characteristics of political cities and underdeveloped commercial trade have also made the city mainly manifest itself as the center of social material wealth and consumption center. Thirdly, the urban population is relatively concentrated, but at the initial stage of urban-rural differentiation is not very distinct, and the degree of population density is not an absolute indicator to judge whether a city is a city or not [5]. Wu, Liangfu proposed seven urban standards from the perspective of urban planning. 1) Cities gather a certain number of people. 2) Cities are mainly non-agricultural activities, which are different from rural social organizations. 3) Cities are the functions of different regional centers in politics, economy and culture. 4) Urban requirements are relatively centralized to meet the needs of residents in production and life. 5) Cities must provide necessary material facilities and strive to maintain a good ecological environment. 6) Municipality is a social entity that coordinates the operation according to the common social goals and various needs. 7) Cities have the mission of inheriting traditional culture and developing it continuously [6]. Mao, Xi also put forward ten criteria for the formation of a city. He believed that if only two thirds of the criteria were met, it could be judged as a city. The criteria are as follows. First, political, religious and cultural centers are formed in the region. Second, emerge of settlements that military defense functions have been strengthened, and many of which are marked by the construction of city walls. Third is the formation of the state and civilization. Fourthly, the number and density of population agglomeration are larger than those of the villages in the region. Fifth, the composition of population is different from that of the countryside. Sixth is the center of wealth accumulation and consumption. Seventh, large settlements and buildings are appeared. Eighth is the emergence of metal objects. Ninth, invent of writing and the formation of science. Tenth, form of markets and trade. In my opinion, because of the different reasons for the formation of various cities, their standards are also different, but the following two conditions must be concluded. First, the number and density of population aggregation are greater than that of the adjacent areas. Second, it should be the political, economic and cultural centers in a certain region.

Reasons for the Formation of Cities
According to the definition and formation criteria of the cities above and the historical materials excavated by archaeology, it can be inferred that cities first appeared in the Mesopotamia (about 3500 BC), the Nile River Basin (about 3500 BC), the Indus River Basin (about 2500 BC), and the Yellow River Basin (about 1500 BC) [7]. Liu, Tao [8], Wang, Shengxue [9], Chen, Heng [10], Chen, Chun [11] and so on analyzed the reasons for the emergence of cities, which mainly include the followings. First, because of the surplus of agricultural products in cities, it can support non-agricultural population such as officials, craftsmen and businessmen, and eventually form cities, such as the capital of Aztec, South America [11]. The second, cities formed by the development of handicraft industry which can absorb more people to form cities, such as Jingdezhen in China [12]. Third, cities formed by commodity exchange that objectively requires a stable place and market to gradually develop into a city, such as Quanzhou, China [9]. Fourth, cities are formed by the promotion of religion. The scale of religion may affect the development of cities, such as Mecca [11]. Fifth, cities are appeared because of military defense. Due to the need of military defense, each tribe builds defensive fortifications around its own residence and gradually forms a city, such as Jiuquan in China [12]. Sixth, Cities are formed for political reasons. The rights of rulers centralize financial and material resources to form cities in the region, such as ancient Rome [11]. The above analysis can provide some effective methods for rural areas to seek the suitable way of urbanization.

Conclusion
In this paper, the definition and formation of cities are sorted out and summarized, which can provide reference for rural areas to realize local urbanization by using their own advantages, and effectively promote the process of urbanization. However, in order to formulate specific countermeasures for urbanization de-velopment in a certain region, we still need a further study in its own conditions in depth and fully, and draw on the advanced experience of each region.

Conflicts of Interest
The author declares no conflicts of interest regarding the publication of this paper.